1. Technical Field
The present invention relates generally to semiconductor processing, and more particularly, to end point detection in semiconductor processing using image matching.
2. Related Art
Chemical Mechanical Polishing (CMP) is a well known process for polishing semiconductor wafers. In general, a semiconductor wafer is placed in frictional contact with a moving polishing pad to remove material from the surface of the wafer. The material is typically removed until a desired thickness of the semiconductor wafer is achieved and the polishing process is then terminated. The determination of when to terminate the polishing process is referred to as endpoint detection (EPD).
EPD may take a variety of forms all with the ultimate goal of achieving uniformity among different semiconductor wafers even though process and material variations continuously occur during the polishing process. One category of EPD techniques are known as “in situ” techniques. In situ end point detection operates “on the fly” during the polishing process to measure the semiconductor wafer. The measurement may involve electrical measurement techniques based on capacitance, impedance or conductivity of the wafer. Another measurement technique involves detection of light reflected from the surface of the in-process semiconductor wafer. The reflected light in the form of spectral data is captured and the intensity measured to arrive at an intensity value. The intensity value is indicative of the thickness of the wafer.
A detailed discussion of endpoint detection with a spectral signal is described in international PCT application PCT/US00/12776 published under publication number WO 00/67951. In general, one or more reference semiconductors are polished to a desired thickness and measured to determine a threshold spectral value. During subsequent polishing of other semiconductor wafers, the threshold spectral value is compared with an endpoint signal. For dielectric processing, the endpoint signal is determined from a mathematical model using a spectral value from the semiconductor wafer being polished. For metal polishing, an amplitude ratio that is determined based on division of the amplitude of two pre-selected wavelength bands is used to determine the endpoint signal. When the endpoint signal reaches the threshold spectral value, polishing is terminated.
One drawback to this form of endpoint detection is the significant amount of processing required to translate spectral data to a single value that is an endpoint signal. In addition, due to averaging, ratios and/or other processing of the spectral data to obtain the endpoint signal, accuracy may be compromised. Further, the granularity of repeatable endpoint detection may be undesirably large due to representation of the spectral data with a single value. End point detection based solely on spectral data may also not optimize accuracy in some applications.